The potential for the change
from cobaltous to cobaltic ion is —1.84 volts. This is strong enough to oxidize
water
with the formation
of oxygen. There are not many examples of cobaltic ions; CoF3 and Co2(SO4)3
• 18H2O have been formed, but they decompose in aqueous solution to
give cobaltous ions. When the cobaltic ion unites with other ions to form com­plexes,
it can be stabilized. Examples of this are Co(CN),r3 and Co(NH3)J3.

Nickel. Nickel, like cobalt,
is difficult to extract in the pure state from its mineral source. The name
comes from the Swedish word, kopparnickel, which means false copper. It
is found in the form of a sulfide mixed with sulfides of iron, or as a silicate
mixed with magnesium. It is normally purified electrolytically; but there is an
unusual method available employ­ing a volatile compound, called nickel
carbonyl, Ni(CO)4, that nickel forms with carbon monoxide at room
temperature. When the vapor of this substance is heated to 200° C, it decom­poses
into nickel and carbon monoxide. Of the three members of this iron triad,
nickel is the least subject to oxidation; for this reason, nickel-plating of
iron produces resistance to corro­sion. It also increases the strength of iron
when present in small amount. A great deal of nickel is also used in coins. In
practically all nickel compounds, the Ni atoms have lost only the two s
electrons in the fourth level and are in the +2

oxidation state. The oxide in
this state is NiO, nickelous oxide, which is black in color The nickelous
complex ions are quite stable in air with respect to oxidation; they can be
either square-planar or octahedral in shape: Ni(NH.,)+2 and Ni(NH3)+2;
and other ions are found with tetrahedral coordination: Ni(CO)4.
Solutions of nickelous ion are usually pale green, but in the presence bf
complexing agents the color changes; for example, in the presence of NH3 nickelous solutions are blue.
Nickelous oxide combines with oxygen to give a substance with the rock salt structure
that has a larger percentage of oxygen than the formula, NiO, indicates; it is
believed that this is the result of a complex
solid solution in which some of the nickel ions are 1 present as
nickelic ions and some nickelous ions are missing from their lattice positions.
Nickel has a boiling point almost the same as that of iron and a melting point
slightly lower; its ionization potential is
the lowest of the three members 1 of the triad, 7.6 volts, and its
density is the highest, 8.9 gm/cm3. It also has the lowest oxidation
potential %r the change from the metal to the nickelous ion,
0.25 volt.